I don't believe you can "change" the past... you can only "view it".
Actually, you kind of can, but it's nothing like "Back to the Future" or "Butterfly Effect". You can go back and re-experience things from your own past, and so something differently or just "deflate" the event of energy. Doing this will definitely lessen or even remove its effect on you, and in some cases, it will (apparently!) remove memory of the event from other people's realities, as well. This may or may not be actually "changing the past", but it can certainly help deal with a lot of baggage and past memories and so forth. It even works for traumatic memories, though it take a lot of courage and energy to go back into a situation where you were traumatised.
I don't think you can undo serious consequences, though. For example, if you commit a crime and go to prison, I don't think you can go back and change it so that you're not in prison. Well.... maybe some people who are extraordinarily adept at reality creation might be able to. I don't know anyone like that, though (or if I do, they're not fessing up
In my opinion, the Astral is simply your subconscious...
Agreed, though I suspect it might be a shared subconscious, as well, because it can have a sort of "ripple effect" if you do stuff there.
or if you can change it, it's only in your subconscious, not the physical reality.
I mostly agree with that as a way of looking at it, but I've seen examples where doing this actually changes other people's perceptions/memories, as well. I also don't really buy into "physical reality" as one, single, unified "place" or "plane" where everyone experiences the same thing. I have plenty of reason to think that everyone has their own version of it, though in most cases, there's a great deal of "overlap" because for the most part, people use the same basic "templates" for it.
But that's probably another discussion.